Marine and industrial (M&I) gas turbine engines are either developed specifically for these applications or are derived from aircraft gas turbine engines. The engines typically include a core engine having a compressor, combustor, and high pressure turbine (HPT) driving the compressor for generating hot combustion gases which flow downstream to a free wheeling power turbine. The power turbine is joined to an electrical generator and is operated at a synchronous speed such as 3,000 rpm or 3,600 rpm for generating electrical power at 50 Hertz or 60 Hertz as desired.
Large industrial turbines are configured for generating electrical power in the range of about 100 megawatts (MW) to about 200 MW. Such high electrical power output may be obtained from a gas turbine by injecting steam into the free wheeling power turbine as is conventionally known for increasing the specific horsepower thereof by a factor of about 5 for example. In this way, the overall size and therefore complexity and cost of the turbine may be contained while still producing a substantial increase in horsepower for driving larger generators than would otherwise be possible. However, one significant problem in generating the high specific horsepower from the free wheeling power turbine is an attendant increase in rotor thrust loads or forces which must be accommodated by thrust bearings. State of the art thrust bearings for an output shaft of about 40 cm outer diameter for a 3,600 rpm synchronous speed are limited to about 41,000 kg thrust loads. Conventional thrust bearings such as the Kingsbury type sized for these larger thrust loads require a substantial mount of oil flow which oil flow experiences heating due to friction losses therein and result in a more complex and costly system. However, the thrust loads for a 200 MW power turbine are projected to be substantially greater and on the order of about twice the present design limit capabilities of conventional thrust bearings, for example over 100,000 kg.
Another significant problem in large steam-injected power turbines operating at 3,600 rpm is the substantial centrifugal stresses generated in the power turbine rotor blades since such blades are relatively long for extracting the required power from the combustion gases flowing therebetween. Conventional stationary stator vanes in the power turbine may typically require cooling whereas the rotor blades may be typically uncooled, with cooling sir being bled from the compressor of the core engine for example which not only decreases overall operating efficiency of the engine, but the spent cooling air from the vanes dilutes the combustion gases in the power turbine which decreases their efficiency of boiling water in a cooperating boiler disposed downstream from the power turbine.
In view of these exemplary considerations in using a gas turbine engine for powering an electrical generator for producing relatively large output electrical power, the design thereof becomes relatively complex and therefore costly.